U.S. Pat. No. 5,036,341 describes a method and device for generating images on an information carrier, such as paper, by means of an array of control electrodes located between a toner carrier member, so-called developer roller and a back electrode. The control electrode unit consists of a woven mesh of wire electrodes where the spaces between wires become apertures through which toner particles are attracted from the developer roller towards the back electrode. By connecting the control electrodes wires to selectable potentials, the apertures can at least partly be opened or closed electrostatically to passage of toner particles.
U.S. Pat. No. 5,121,144 describes another control electrode unit consisting of a thin insulating substrate with apertures. The apertures are surrounded by ring electrodes on one side of the substrate.
Other types of control electrodes are also known, for example as in UK 2 108 432 where electrodes are located on each side of an insulating substrate. Ring electrodes or the like, at least partly surround each aperture on one side of the substrate while a common electrode surrounds all apertures on the opposite side of the substrate.
FIG. 1 shows, in a schematic way, a cross-section view of a printer unit 10 according to U.S. '341 and '144. The developer roller 12 rotates in a toner container (not show) and attracts toner particles 11 to the roller surface by means of magnetic or electrostatic forces. Toner particles 11 are arranged in a thin layer on the developer roller 12, whose surface may be an electrically conducting or semiconducting material. An electrostatic field is established between the developer roller and a back electrode 15 by for example grounding the developer roller and connecting 1500 volts to the back electrode. That electrostatic field will transport toner particles from the developer roller through the apertures 17 to the surface of an information carrier 13. A control potential of for example -200 volts connected to the control electrodes 16 of an electrode unit 18 will modify the electrostatic field at the developer roller in the region of the control electrode, closing the aperture 17 to passage of toner particles. A control potential of for example +150 volts will modify the electrostatic field at the developer roller in the region of the control electrode, opening the aperture to passage of toner particles from the developer roller through the aperture to the information carrier 13.
Use of a cylindrical developer roller to bring toner particles close to the planar control electrode array causes the distance l.sub.k between the developer roller and each control electrode to depend on the location of the control electrode within the control electrode array. The l.sub.k distance for aperture A1 for example is less than the l.sub.k distance for aperture A4. The variation of l.sub.k distance among the apertures is represented by .DELTA.l.sub.k. Variation of the l.sub.k distance among the control electrodes causes a variation in the electrostatic field for attracting toner particles from the developer roller. An approximate relation of control electrostatic field to the l.sub.k distance is shown in FIG. 2. Variations of the l.sub.k distance cause variations in the control electrostatic field that causes variation in the number of toner particles attracted to the surface of the information carrier.
Those variations of toner particles cause undesirable variation in the printed image.
A means of charging and transporting toner particles is needed that can be made coplanar with the control electrode array so that the l.sub.k distance is more uniform.